Abstract
In the preceding paper (which we shall refer to as I) Teller and one of us (H.) have discussed some characteristic effects occurring at the low temperatures which can be attained by the magnetic cooling method. In this method a paramagnetic salt is used, the spins of which can be freely, or almost freely, orientated in an external magnetic field. It is essential, however, that (at least at the initial temperature T
i
) the magnetic energy H of the spins can be transferred to the lattice vibrations. For this purpose the interaction between the spins and the lattice vibrations is essential. Now it has been shown in I that this interaction is in general very small. It depends upon the temperature and the magnetic field H applied and is practically interrupted when the lattice vibrations have reached a critical temperature T
c
given by I equations (14) and (13). Under usual conditions (H∽ 15,000 gauss) T
c
is of the order of 0·1° abs. The crystal can then be considered as consisting of two practically independent systems, the spins of the paramagnetic ions and the lattice vibrations. If a temperature lower than this critical temperature T
c
is measured, the temperature refers to the spin system only, but not to the lattice vibrations. When the field is switched off again the thermal equilibrium between the spin system and the lattice will be restored very slowly, the time required being of the order of an hour at 0·2° abs (
cf
. I, equation (15)).
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